High quality image acquisition in electron microscopes requires careful alignment of the electron beam and precise focusing for optimized image contrast and fine detail. In the past, physical features and characteristics of the electron-microscope have been important and used to perform the alignment. The electron beam is aligned by using electromagnetic devices. A misaligned electron beam results in artifacts (ripples), blurriness in the image, and loss of information on fine details.
An important feature of the present invention is that the method automatically corrects for lens astigmatism during the alignment process by using only image data and without relying on complicated and cumbersome features of the microscope itself. The method of the present invention provides a solution to the above-outlined problems. More particularly, the method is for automatic astigmatism correction in one direction through a set of lenses. Of course, the present invention is not limited to correcting in only one direction because the correction can also be done in many directions simultaneously such as both the x- and y-directions. A first image is provided at a first stigmator setting of a lens. Preferably, the image is under-focused. Based on the first image, a calculating device calculates a first Fourier spectrum image. The distribution and direction of pixels of the Fourier spectrum image are determined by calculating a first vector corresponding to the main direction and extent of the bright pixels, and a second vector being perpendicular to the first vector and corresponding to the extent in that direction. The first vector is compared with the second vector. The set of lenses is changed from a first stigmator setting to a second stigmator setting to provide a second under-focused image. Based on the second image, the second corresponding Fourier spectrum image is calculated. The distribution and direction of pixels of the second Fourier spectrum image is determined by calculating a third vector and a fourth vector. The third vector is compared with the fourth vector. When the first vector is more similar to the second vector than the third vector is to the fourth vector the intensity distribution in the first image is selected as being rounder than the second image. When the third vector is more similar to the fourth vector than the first vector is to the second vector then the intensity distribution in the second image is selected as being rounder than in the first image. The stigmator settings providing the Fourier spectrum with the roundest Fourier spectrum is what is strived and searched for.
The method further includes the step of calculating grey-weighted moments of a circular Fourier spectrum image as a means of measuring the direction and extent of the intensity distribution.
In another embodiment, a first ratio of eigen-vectors of the first Fourier spectrum image is compared with a second ratio of eigen-vectors of the second Fourier spectrum image.
The image with the lowest ratio is selected.
The x-stigmator and y-stigmator settings are changed to the stigmator settings that correspond to the image with the lowest ratio.
The x-stigmator and the y-stigmator settings can also be simultaneously changed.
In yet another embodiment, the stigmator setting that minimizes the elongation value of the Fourier spectrum image is searched for.
The first and the second images are set to an under-focus or an over-focus.
In an alternative method of the present invention, the following steps are used for automatic astigmatism correction of a lens system:
A first image of a first frequency spectrum in a microscope is provided. The first image of a frequency spectrum of a view that is not in focus at a first stigmator setting of a lens. The first image is imaged. A first roundness measure of a distribution and directions of intensities in the first image is determined. The lens is changed from the first stigmator setting to a second stigmator setting to provide a second image of a second frequency spectrum. The second image of the view that is not in focus at the second stigmator setting of the lens. The second image at the second stigmator setting is of the same view as the first image of the view at the first stigmator setting. A second roundness measure of a distribution and directions of intensities in the second image is determined. The first roundness measure is compared with the second roundness measure. When the first roundness measure indicates a rounder distribution than the second roundness measure, the first image at the first stigmator setting is selected, and when the second roundness measure indicates a rounder distribution than the first roundness measure, the second image at the second stigmator setting is selected.
The method further includes the step of imaging a back-focal plane at a first stigmator setting and imaging the back-focal plane at the second stigmator setting.
In another embodiment, the method further includes the step of setting the microscope to a diffraction mode.
In yet another embodiment, the method further includes the step of calculating a first vector and a second vector and comparing the first vector with the second vector to determine the first roundness measure.
In another embodiment, the method further includes the step of calculating a third vector and a fourth vector and comparing the third vector with the fourth vector to determine the second roundness measure.
In yet another embodiment, the method further includes the step of selecting the first image at the first stigmator setting when the first vector is more similar to the second vector than the third vector is to the fourth vector, and selecting the second image at the second stigmator setting when the third vector is more similar to the fourth vector than the first vector is to the second vector.